Another "revolutionary new" hull design

Here a magazine tested same boat, Minor 27, with 3 different propulsion arrangements, a jet, inboard, and a leg. I think the leg was Bravo2 and all had a very similar Yanmar diesel. Unsurprisingly the leg was more efficient over an inboard by quite a large margin, but not this much. The jet started to bypass others after 30kn.

 

I think its a bunch of marketing bull:

That is not the purpose of a stepped hull. the marketing department needs to communicate with the designer before they print this drivel.

A keel pad does not add directional stability, in fact they can make a boat more unstable as the vessel transitions off and on the pad. Looking at the rendering what I am seeing is not a keel pad. A keel pad is flat, or nearly flat. If a keel pad runs all the way to the front, there may be greater deadrise towards the front, but would quickly transition to a nearly flat pad aft, thats not what I'm seeing

This statement is quite certainly not true. Although experiments have shown that microbubbles introduced into the turbulent wetted boundary layer of flat plates do show a reduction in skin friction; however in full scale tests, there was actually a net power loss. Studies have shown that microbubbles are most effective at about 13 knots and gains quickly deteriorate at higher speeds. This boat is barely on plane at 13 knots, so doubtful there is much air being drawn under the hull. In addition the hull is to short for any significant benefit from microbubbles. Most probable is that any air under the hull is going to reduce the propellers effectiveness.

The company claims "maximum fuel economy" and "superior performance"; a top speed of 24 knots at 2.5 nmpg is neither. At 260HP, the boat should reach speeds between 30 and 33 knots depending how it is loaded. More in line with Will's Graphite.

It reminds me of similar nonsense as that printed on the Axcell website.
http://www.axcellyachts.com/?page_id=485

 

What I see in this "revolutionary" design is a heavy boat, and a noisy boat. Ill bet that you can hear that boat moving thru the water a mile away.

Also all those chines will present a maintenance nightmare.

Anti foul doesn't like sharp 90 degree angles which comprimise film thickness of the paint...the vessel will foul much sooner than a smooth bottom.

I do like the robust skeg.

 

Yup --Fred, you're right there-- Recall some of those early Tiawan vessels, newspaper cores was one of their big tricks--

 

Call it what you will... I'm happy with the term skeg...
There are applications where the use of a shaft makes more sense than a sterndrive, I agree. I don't think this is one of them. As a boat intended to be trailerable, the draft of 0.7m will be restrictive in terms of where it can easily be launched to start with. It will also increase the air draft whilst on the trailer.
The increased protection afforded by the skeg is advantageous, as is the increase in lateral plane area (one of the disadvantages of a very shallow draft vessel). But the increased efficiency more than offsets any perceived increase in maintenance costs IMHO. Yes, a sterndrive will tend to cost more to look after than a simple shaft and prop, but as long as you look after them, the costs are relatively minor in the overall scheme of things.

Not only that, the reduced draft allows access to areas that shaft-driven boats simply can't go; There's an increase in range afforded by the increase in efficiency.... the list goes on....

A sterndrive will typically be around 20% more efficient than a traditional shaft setup at lower speeds. Beyond 30 knots jets tend to be more efficient, as you say (but woefully inefficient at lower speeds). Beyond 40 - 45, surface drives are generally the most efficient... but again, not so good going slower....

 

I know this isn’t a propulsion thread, but since we’re talking about the pros/cons and effectiveness of the skeg on this boat I thought I would include the following link that shows actual performance differences from 4 different propulsion setups on a boat similar in length and displacement to the Cutwater. The following article tested 4 propulsion setups on the same Hunt CC hull: diesel waterjet, diesel sterndrive, gas sterndrive, diesel inboard. Some points I would like to point out. The diesel waterjet was less efficient on all points including top end. The diesel sterndrive was more efficient on all points but only by about 9% at cruising speed. (over the inboard with same motor) The diesel inboard was quieter than all other choices at cruise speed. One more notable design point; the diesel inboard was less efficient in part for this particular aplication due to the fact that the inboard setup moved the Cog further forward. The inboard runs with the least angle of trim, and therefore the most wetted surface area forward.

http://www.huntyachts.com/documents/soundings-article-20080801.pdf

 

What ever turns your prop --The point i was trying to make was shaft drive verses the typical outdrive which have a history of high maintenance costs due to many moving parts and casings exposure to salt water corrosion. I agree the shaft set up is more subject to launching and haulout damage but as a rule shaft setups are not generally looked at as your typical trailerable class. More intended to be launched and hauled out on a seasonable basis. Yes we agree to agree. Being old school when i see a shaft set up pertruding from an appendage, I automatically think keel . Personally from first hand knowledge of many years in boat building and maintenance I wouldn't own anything else although I know outdrive setups do have their place. If I were a customer of the craft under discussion the shaft setup would be a plus. Then again I'm not a trailer boater.

 

I wouldn't have another inboard trailable boat as the boat has to be higher off the ground when on the trailer, than an outboard.

I have to push mine in the water halfway across the Indian Ocean to get it off and on the trailer.

Poida

 

Inboard with shaft certainly a big minus as a daily trailerable craft and one in which an outdrive set up is big plus whether it be an inboard/outdrive or a regular outboard.

 

MOB - thanks for the link to the Hunt 25 test. Makes for quite interesting reading. As you'd expect from one of the foremost design groups, they've done an exceptional job - especially with the shaft drive. I don't think I've ever seen a shaft installation that gives so little away in terms of efficiency. Shame the test didn't also include the outboard powered version that Hunt produce... then we'd have just about everything.
Just for interests sake I plotted the consumption against speed for the various Hunts, Graphite and the single point available for the Cutwater. Of course, it can hardly be considered definitive... there is precious little info about the test conditions for the Hunt... I expect they were conducted in relatively light trim, with just one onboard. Graphite's numbers were produced with 50% fuel, 70% water, 2 crew and in typical cruising condition... I expect she would weigh in at close to 1000kg more than the Hunt at the time.
The most interesting thing from my point of view is the difference in consumption at approx 7 - 13 knots - typical "hump" speed. This was one of my primary objective's in the design of Graphite - to allow the operator to run at whatever speed the conditions predicate. Of course, it has far less deadrise than either the Hunt or the Cutwater, so one would expect to have to slow down more... but it's a cruising boat, not a fishing boat... the intent is to potter about comfortably, not belt about with a bunch of fishing mates...

Apologies for the side-track.... But I do think it demonstrates one thing clearly... there's nothing revolutionary about the original vessel in question...

 

Thanks for the chart. You might want to consider labeling the vertical axis as "Efficiency" rather than "Consumption". When I see consumption I assume a larger number means more fuel is consumed.

 

Having trouble....

Having trouble seeing how this design can be positive, meaning it looks like it gains substantially less than it loses.

 

Yes... good point... did it in a hurry.... will edit it when I get a spare moment....

 

i noticed earlier in the thread that the graphite had a "displacement of 2900kgs" whereas this cutwater states a "dry weight of 2900kgs" ie. the graphite is actually a MUCH lighter boat... i doubt it needs it as the difference is quite apparent simply from the photographs, but can you please clarify? To put apples to apples, what is the dry weight of the Graphite?

 

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